Magnetic stripe data transmission or magnetic security transmission (MST) is a technology that magnetic signals similar to magnetic stripe data of a traditional payment card are transmitted from a transmitter to a receiver by an MST driver. The transmitter may be a host device such as a smart-phone. The receiver may be a payment terminal's card reader. The magnetic signals emulate magnetic stripe data of the payment card that are normally read by a card reader while physically swiping the payment card on a reader head.
FIG. 1 shows a schematic representation of an inscription of payment card data on a magnetic stripe of the payment card. Waveforms corresponding to the magnetic stripe data are picked up by the payment terminal's card reader's head while swiping of the payment card along with digital equivalent of the waveforms. The MST driver emitted magnetic signals emulate the same waveforms at the payment terminal's card reader without swiping of the payment card.
In conventional magnetic stripe data transmission or magnetic security transmission (see U.S. Pat. No. 8,814,046), the MST driver is configured to transmit the magnetic strip data comprising streams of pulses. The MST driver preferably includes a full bridge type switch configuration connected across a voltage source and a ground to drive bidirectional load current through an inductive coil according to the magnetic strip data. The MST driver transmits the magnetic signal to the card reader. In the transmission of the magnetic signal, magnetic flux density of the inductive coil is varied according to the load current density, inductance value and the load current slope of the inductive coil which remotely induces a back electromagnetic force (Bemf) in a receiver of the card reader. If the back electromagnetic force (Bemf) is higher than a threshold value, the card reader then recognizes it as a High pulse. If Bemf is lower than another threshold value, then the card reader recognizes it as a Low pulse. The High and Low pulses in combination can re-construct the card reader's read head waveforms.
FIG. 2A shows a circuit representation of the MST driver. The MST driver comprises four MST driver switches 101, 102, 103 and 104 arranged in a full bridge type configuration connected across a voltage source and a ground VM 108. An MST coil 105 is modeled by its inductor 106 having inductance L1 and series resistance R1 107. Each of the MST driver switches includes a respective body diode (D1-D4) connected across said each switch that plays a role of free-wheeling current path of stored energy in the inductor 106 during a switch off period.
The MST driver switches 101, 102, 103 and 104 are driven by an external or a built-in driving integrated circuit (IC). They have pulse shaped driving waveforms with usually 50% duty ratio of a constant frequency or a doubled frequency. In the MST driver, both the first 101 and the fourth 104 switches are simultaneously turned on for driving the load current in the MST coil 105 in a forward direction. Both the second 102 and the third 103 switches are simultaneously tuned on for driving the load current in the MST coil 105 in a reverse direction.
FIG. 2B shows an MST driver's switch driving operation and a corresponding load current waveform. The waveform can be divided into 6 time durations, T1, T2, T3, T4, T5 and T6. Time durations T1, T2 and T3 may be forward driving periods. The load current is positive during time durations T1, T2 and T3. Time durations T4, T5 and T6 may be reverse driving periods. The load current is negative during time durations T4, T5 and T6. The positive or negative value is entirely based on a designer's perspective.
In the FIG. 2B, the first 101 and the fourth 104 switches are turned on. The load current increases during T1 period. It reaches a positive peak current. In T3 period, the first 101 and the fourth 104 switches are turned off and then the second 102 and the third 103 switches are turned on. The load current starts decreasing abruptly but is still positive. It is called a reverse braking. With the second 102 and the third 103 switches in a turned-on state, the load current becomes negative in the T4 period. During T4 period, the load current slope and the absolute peak value are the same as T1 except that they are in opposite directions and are negative values. During T5 period, the negative peak current continues to flow. In T6 period, the second 102 and the third 103 switches are turned off and the first 101 and the fourth 104 switches are turned on. The load current begins to fall abruptly and has the same slope as T3 except that they are in opposite directions.
FIG. 2C shows switching cycles of the MST driver switches, the corresponding load current waveforms in the MST coil and induced back electromagnetic force (Bemf) at card reader's receiver. When the first 101 and the fourth 104 switches are driven by the same signal to turn on, the load current IL through the MST coil 105 starts to increase from previous current and reaches the peak current Ip. The peak current Ip is dependent on a supply voltage of a voltage source VM 108 and the total series resistance R1 107 of the MST coil. It can be represented as
      V    M        R    1  if the switches' on-resistance is ignored. The load current
  (            I      L        =                  I        P            ⁡              (                  1          -                      2            ⁢                          e                                                -                                                            R                      1                                                              L                      1                                                                      ⁢                t                                                    )              )increases exponentially with the power of
      -                  R        1                    L        1              ,where L1 is the inductance value of the MST coil. Similarly, if the second 102 and the third 103 switches are driven by the same signal to turn on, the load current
  (            I      L        =          -                        I          P                ⁡                  (                      1            -                          2              ⁢                              e                                                      -                                                                  R                        1                                                                    L                        1                                                                              ⁢                  t                                                              )                      )through the MST coil 105 starts to decreases exponentially with the power of
  -            R      1              L      1      from the previous current and reaches −IP.
In the FIG. 2C, a first (I) and a second (II) transient instant of the load current contribute to the magnetic signal transmission, since the induced Bemf reaches its peak value during transient variation of the load current depending on the load current slope in the Bemf waveform of FIG. 2C. The steady state periods of load current fixed to +Ip or −Ip have no contribution to induce Bemf. If the induced Bemf generates a voltage signal higher than a positive threshold voltage Vr on the receiver in the card reader, the card reader recognizes it as “High”. If the induced Bemf generates a voltage signal lower than a negative threshold voltage −Vr, the card reader recognizes it as “Low”.
The back electromagnetic force (Bemf) depends on the magnetic flux density change ratio which follows current density change ratio in the inductive coil. The current density change ratio to time is basically the load current slope which is inversely proportional to the inductive coil's inductance value. In a fast current slope, the induced Bemf is big. In a slow current slope, the induced Bemf is small. In a fast current slope, if the corresponding duration is too short the receiver in the card reader may not recognize the signal. In a fast current slope with long duration, a peak inductive current increases. It may exceed current rating of the MST driver. It causes additional power loss by high current. The high current slope has side effect, for example, noise and Electromagnetic Interference (EMI) issues.
Optimization and control of the load current slope and time duration are important in the MST driving technology so as to ensure reliable signal transmission while consuming less power. However, in the conventional MST driver, the load current slope cannot be controlled except changing parameters including coil's inductance, series resistance of the coil or on-resistance of the full bridge driver's switches. It may not be easy to control those parameters because limiting factors have trade-offs in performance, cost and form factor. One way is to increase the inductance of the MST coil, but bigger inductance requires larger size and increased cost. Therefore, prior art MST driver cannot deliver. The prior art MST driver has low energy efficiency due to limited inductance. It requires long duration to achieve good transmission quality. It may lose signal because of increased efficiency.
The performance of the prior art MST driver is affected by the power supply voltage and the MST coil because it is difficult to control or adjust them. In term of efficiency, the prior art method consumes a lot of power even during time periods without signal transmission. The signal transmission is done only in the transient period of the load current. The steady state of the peak current consumes power without conducting work. It is much longer than the transient time. Energy efficiency is much worse. It has a big impact on a power supply system.
It has a need to develop a new MST driver that can program or control the load current slope value and time durations to ensure reliable signal transmission with less power consumption.